1. Field of the Invention
The present invention relates to a light beam scanning apparatus and a method for sweeping and scanning a light beam such as a laser beam on an object surface.
2. Description of the Prior Art
With the recent advancement of laser technology, a scan system of an object by a laser beam has been utilized in the field of instrumentation. The laser beam has an advantage that even a small power beam can be focused to a small diameter spot having a high light intensity.
FIGS. 1A to 1C show a prior art laser spot scanning system which uses a conventional simple scanning lens in a surface defect detecting apparatus for detecting defects or dusts on a surface of a silicon wafer of a semiconductor device. A laser beam 1 is angularly swept within an scanning angle of .+-..theta. by a scanner 2 such as a vibrating mirror and focused by a scanning lens 3 so that a line 5 is scanned by a laser spot on an object 4. By moving the object 4 in a y-axis direction while scanning the laser spot in an x-axis direction, a whole area of the object 4 is scanned.
An optical lens usually includes more or less aberrations such as spherical aberration. When such an optical lens is used as the scanning lens 3, a variation of a spot diameter or a distortion of a spot shape takes place depending on the scan angle .theta. or a spot position. FIG. 1C shows a curvature of field which results in the variation of the spot diameter. As the scan angle .theta. increases, a focusing point Q of the laser beam is moved toward the light source away from the object plane 4 so that a locus 6 of the focusing point Q is curved. As a result, the spot diameter d.sub.c of the spot P on the object 4 becomes larger than that when the scan angle .theta. is zero. That is, a variation takes place in the spot diameter d.sub.c.
While the variation in the spot diameter in the scanning direction (x-axis) has been discussed above, a variation also takes place in the direction (y-axis) orthogonal to the scanning direction. As a result, a circle of confusion is produced on the object 4. Since the degrees of variation in the x-axis direction and the y-axis direction are usually different, the circle of confusion is usually non-circular.
The variation of the spot size naturally results in a variation of light intensity. Thus, in a system for measuring a size of a defect by measuring a light intensity of a scattered light resulting from the defect, defects of a same size will be detected as of different size depending on the position of the spot on the object 4.
The size of the object 4 such as a silicon wafer has been increased from several tens mm in diameter in an early stage to 100 mm in diameter in present days and will be increased to 125 mm in diameter in near future. On the other hand, in the laser spot scanning system shown in FIGS. 1A-1C, when the spot size is approximately 15 .mu.m in diameter, an allowable scan range having a small variation in the spot diameter may be only 20 mm at most. Thus, in the prior art, a zone-split scanning system is used in which the object 4 is split into a plurality of zones and the respective zones are sequentially scanned to complete the scan or detection of the whole area. Thus, a long inspection time is required. This has been a neck in a production line.
Thus, an optical lens which results in a small variation in the spot diameter over a wide scan width has been desired. A scanning lens called an "f.theta. lens" has been known which aims to improve an f-.theta. characteristic, that is, a linearity of a spot position to a scanning angle .theta.. However, it does not always provide an optimum condition to the variation of the spot diameter and the shape of the circle of confusion and complex in construction. In addition, since a compound lens system including the f.theta. lens is expensive, it is not suitable to use in the surface defect detecting apparatus which requires a large diameter lens.
A scanning system which uses a concave mirror has also been known. The concave mirror is much more inexpensive than an optical lens of the same diameter and can provide a scanning system having a satisfactory spot size and variation of the spot size by appropriately selecting parameters of the optical system such as an incident angle. Japanese published patent application No. 55-36127 entitled "Optical Scanning Apparatus" discloses the use of a parabolic mirror as the concave mirror and the arrangement of a scanner at a specific position in accordance with a focal distance f of the parabolic mirror in order to minimize the aberration such as the circle of confusion.
FIGS. 2A and 2B show a construction of the light beam scanning apparatus in accordance with the above-mentioned patent application. A parabolic mirror 7 having a focal distance f is arranged to oppose to an object 4 and a scanner mirror 2 is arranged at a specific position between the parabolic mirror 7 and the object 4. The laser beam 1 is angularly swept by the scanner 2 and reflected and focused by the parabolic mirror 7 so that a spot P having a very small circle of confusion scans the surface of the object 4.
In order to prevent the scanner 2 from interfering an optical path of the beam reflected from the parabolic mirror 7, an optical path from the scanner 2 to the parabolic mirror 7 is inclined by an angle .OMEGA..sub.o in a y-axis sectional plane. The scanner 2 is positioned on a center axis (z-axis) of the parabolic mirror 7 which is normal to the object plane 4 and the optical path or axis is inclined by the angle .OMEGA..sub.o. An effective working range of the parabolic mirror thus does not include a center 0 of the parabolic mirror 7. This is called an off-axis configuration.
Various aberrations or variations (which are hereinafter simply referred to as aberrations) are included in the light beam scanning system. They include the circle of confusion described above as well as the distortion or the non-linearity of the locus of spot which is to be corrected by the f.theta. lens. In practice, depending on the object and the usage of an application apparatus or an instrumental apparatus, one or all of those aberrations must be eliminated. For example, in a facsimile art, the three aberrations mentioned above must be eliminated to maintain a high quality of image. In the surface defect detecting apparatus which the present invention is to be applicable, the curvature of field must be highly eliminated but the distortion and the non-linearity of the locus of spot need not be completely eliminated so long as they are not too large because they can be compensated by a system configuration.
Regarding the aberrations in the light beam scanning system which uses the concave mirror, it is considered necessary for attaining the high performance laser spot scan not to limit the concave mirror to the parabolic mirror and to consider the aberrations for the angle .OMEGA..sub.o of the inclination of the optical path or axis to the concave mirror in the off-axis configuration and to select a light beam scanning system which meets the requirements.